Lens/mirror tower for an optical storage device

ABSTRACT

A lens/mirror tower (LMT) of a multiple-disk array, optical storage system includes a plurality of lens and mirror elements integrally formed on a monolithic, optically-clear substrate. There is a specific lens/mirror set for each recording disk surface of the optical storage device; however, the mirror elements are preferably configured as one reflective surface of the substrate. The lenses are molded onto the opposite surface in a single operation that fixes their relative positions. The LMT is generally fabricated using a lens replication process. Initially, a glass substrate is formed with a trapezoidal configuration. A lens array mold is also formed with a plurality of cavities arranged in overlapping pairs. The cavities are filled with optical-quality resin and the mold is applied to the surface of the substrate opposing the reflective angular, e.g., 45°, surface. Once cured, the resin serves as an array of lenses that are aligned with the optical paths to the objective lenses of the disk array.

This is a continuation of U.S. patent application Ser. No. 07/847,520,filed on Mar. 6, 1992, now U.S. Pat. No. 5,260,928 issued on Nov. 9,1993.

FIELD OF THE INVENTION

This invention relates generally to optical systems and, morespecifically, to a lens/mirror tower arrangement for an optical storagedevice.

BACKGROUND OF THE INVENTION

Conventional optical storage devices typically employ a single opticaldisk having a single recording surface for storing information. Use of asingle disk allows the optical components, such as a mirror and a lens,to be arranged relative to the recording surface in a manner thatoptimizes the size and cost of the device. Although this results in alow-cost device having a relatively small form factor, its storagecapacity is limited to that provided by a single surface. Copending andcommonly-assigned U.S. patent application of Lee et al., for OPTICALSTORAGE SYSTEM, filed herewith, describes a multiple-surface system inwhich the beam from a single laser is steered by a stationarygalvanometer-rotated mirror to optical heads associated with therespective recording surfaces. The heads are mounted on a carriage thatmoves them radially over the surfaces for access to selected data trackson the surfaces.

Specifically, the rotating mirror selectively directs the beam to one ofa vertical array of uniquely oriented deflection mirrors. When adeflection mirror receives the beam, it reflects it along a planeparallel to and close to a corresponding disk surface. The beam thenpasses through an imaging lens on the way to a 45° mirror that redirectsthe beam radially toward an objective lens in the optical headassociated with that surface. The objective lens, in turn, converges thebeam on a small spot on the selected data track.

Therefore, it is apparent that the mirrors and lenses must be preciselyaligned. Moreover, they must occupy a small space so that the formfactor for the overall system is comparable with that of conventionalmultiple-disk, magnetic disk drives. Further, the lens and mirroroptical components must be manufacturable within the cost constraints ofthe conventional single-disk optical components.

Various fabrication techniques might be employed to produce a multiplelens and mirror arrangement. One approach involves the construction oftwo separate components: a multiple mirror component and a multiple lenscomponent. However, the cost of these parts, particularly the lenselements, is relatively high. Fabrication of the separate componentsentails individual assembly, including discrete adjustment and bonding,of each mirror element and each lens element onto respective portions ofa precast housing. Moreover, some of the lenses must overlap because ofthe close proximity of the associated recording disk surfaces. Thisrequires truncation of lenses and fitting together of the truncatedlenses, a costly procedure. The assembly process further necessitatesmanual alignment of the mirror and lens components relative to eachother.

An alternative approach involves plastic molding of an integratedlens/mirror unit. Although this process provides a low-cost unit,current molding technology typically cannot provide the level ofwavelength quality needed for the mirror elements. This deficiencyfurther causes stress in the plastic substrate material and leads tobirefringence of the optical beam when passing through substrate,thereby degrading the accuracy and reliability of the component.

Therefore, it is desirable to provide a reliable, lens/mirror tower foruse in a multiple-disk, optical storage device.

Specifically, it is desirable to provide a tower in which the opticalelements are precisely configured and positioned so that the tower canbe used in a multiple-disk optical storage system.

It is also desirable to provide a low-cost method for producing suchlens/mirror towers.

In addition, it is desirable to provide a fabrication process thatenables installation of mass produced lens/mirror towers in an opticalstorage device without further alignment of the lens/mirror elements.

SUMMARY OF THE INVENTION

Briefly, a lens/mirror tower (LMT) incorporating the invention includesa plurality of lens and mirror elements integrally formed on amonolithic, optically-clear substrate. Specifically, there is alens/mirror set for each recording disk surface of the optical storagedevice. However, in the preferred embodiment of the invention, themirror elements are configured as one reflective surface of thesubstrate. The lenses are precisely molded together onto the oppositesurface in a single operation that fixes their relative positions.

The LMT is generally fabricated using a lens replication process. First,a glass substrate is formed with a trapezoidal configuration; that is,the substrate is a rectangular parallelopiped except for one end surfacewhich is oriented at a 45° angle with respect to the opposite surface. Alens array mold is formed with a plurality of cavities arranged inoverlapping pairs. The cavities are filled with optical-quality resinand the mold is applied to the surface of the substrate opposite theangled surface. Once cured, the resin serves as an array of lenseshaving relative positions that are appropriate for alignment with theoptical paths to the objective lenses of the disk array. Accordingly,the lens replication process provides a fast and low-cost, yet precise,fabrication method particularly suited for high-volume manufacturing.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of the invention may be betterunderstood by referring to the following description in conjunction withthe accompanying drawings, in which:

FIG. 1 is a diagram of an optical beam distribution system for amultiple-disk, magneto-optical storage device in which the method andapparatus of the present invention may be utilized;

FIG. 2 is a perspective front view of a lens/mirror tower (LMT) inaccordance with the invention;

FIG. 3A is a schematic diagram of a preferred embodiment of a substrateused to fabricate the LMT of FIG. 2;

FIG. 3B is a schematic diagram of an alternate embodiment of a substrateused to fabricate the LMT of FIG. 2;

FIG. 4 is a perspective view, partially broken away, of a lens arraymold used to construct lenses during fabrication of the LMT inaccordance with the invention;

FIG. 5 is a diagrammatic view of a mating fixture used to providepressure contact between the substrate and the mold during fabricationof the LMT;

FIG. 6 is a diagrammatic view of the interface between the substrate andmold; and

FIG. 7 is a diagrammatic view of a lens fabricated by a lens replicationprocess in accordance with the invention; and

FIG. 8 is a perspective side view of an LMT fabricated in accordancewith the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 depicts an optical beam distribution system 10 for amultiple-disk array, magneto-optical storage device. The beamdistribution system 10 includes a stationary optics package 12 forgenerating an optical beam and a galvanometer-controlled mirror, i.e.,"galvo mirror" 14, for distributing the beam to one of several opticalhead assemblies 16. Each of the head assemblies is associated with arecording surface of a horizontally disposed optical disk 15 thatrotates about a vertical axis. The galvo mirror 14 distributes the beamto a deflection mirror tower (DMT) 18 having a multi-faceted mirrorsurface 19 arranged to deflect the beam so that it is passed to a headassembly 16 of a selected disk. Specifically, each facet 19a of thesurface 19 is positioned at the same height as a corresponding headassembly 16 and it is oriented so as to reflect the beam from the galvomirror 14 horizontally to a lens/mirror set 30, 32 contained in a lensmirror tower (LMT) 20. The lens/mirror set 30, 32 redirects the beamhorizontally to the corresponding head assembly 16, which redirects thebeam vertically to an objective lens (not shown ).

The LMT 20 includes a plurality of lens and mirror elements integrallyformed on a solid, monolithic, optically clear substrate. FIG. 2illustrates an embodiment of the LMT 20 having a generally trapezoidalhorizontal cross section defining a wedge-like shape. The vertical frontsurface 22 is generally planar so as to provide a foundation for thelenses 30, which are vertically arranged thereto. The back, angular,surface 26 of the LMT 20 is oriented at a 45° angle relative to thefront surface 22 and is preferably coated with a reflective material soas to serve as a mirror. The side 28 includes a plurality of areas thatfunction as datum points 25 to position the substrate in a fixtureduring fabrication of the LMT structure, as described below inconnection with FIG. 5. Additionally, the datum points 25 may be used toaccurately position the LMT within the optical beam distribution system10.

There is a separate lens/mirror set 30, 32 (FIG. 1) for each recordingdisk surface of the optical storage device array; however, in thepreferred embodiment of the invention, the mirror elements areconfigured as the single reflective surface 26 of the LMT. Because ofthe close proximity of disks 15 of the array, the lenses are arranged inoverlapping pairs 34; this intersecting arrangement enables alignment ofthe optical axis of the lenses 30 of a pair 34 with the objective lensesassociated with the closely-spaced recording surfaces of adjacent disks15, as described further below.

In a typical embodiment of the invention, there are five lens pairs 34,for a total of ten lenses 30. Each lens 30 functions as an imaging lensto transfer the reflection of the optical beam at the galvo mirror 14 tothe front focal point of an objective lens. Specifically, the lens 30passes the optical beam to the 45° mirror surface 26, which then directsthe beam to the head assembly 16 containing the objective lens.

In accordance with the invention, an LMT may be fabricated as follows.Initially, a generally rectangular substrate composed of glass materialis constructed. The glass material is preferably boro-silicate crownglass, but other similar materials may be used. Use of bulk glassmaterial improves the optical quality of the substrate and facilitatesbeam passing through the substrate, as compared to plastic molding.Because the LMT is a relatively minute structure having dimensions onthe order of tens of millimeters (mm), the base substrate is preferablylarger than the LMT so that a plurality of LMT blocks may be formed fromthe substrate.

FIG. 3A depicts a base substrate 40 with a width w of 4 mm, a height hof 50.14 mm and a length l of 50.6 mm. The sides 42 of the substrate arepolished to a smooth finish using a conventional optical grinding andpolishing machine. Specifically, the substrate 40 is secured to afixture plate (not shown) and polished with a polishing plate that istypically made of granite. If necessary, the front surface 44 and backsurface 45 are ground to ensure that these surfaces are normal to theside surfaces 42 of the substrate. The substrate 40 is then cut alongits side at a distance l₁ of 27.3 mm from its front surface 44; the cutis made at a 45° angle, as shown in the drawing. This produces twowedge-like, trapezoidal substrates 40a,b, each having an angular surfaceoriented at 45° relative to the front and back surfaces. The front, backand angular surfaces of each substrate are then polished to a smoothfinish. Again, the bulk glass substrate enables use of conventionalgrinding techniques to ensure a precisely flat angular (mirror) surface,which is difficult to attain using current plastic molding technology.

FIG. 3B depicts an alternate embodiment of a base substrate designated50. Here, typical dimensions of the generally rectangular substrate 50are: w=4 mm, h=100.28 mm and l=101.2 mm. Eight wedge-like substrates50a-h are formed by (i) slicing the substrate in half (at a 90° angle)along its side at a distance l₁ of 50.6 mm from the front surface 54;(ii) slicing the resulting two substrates at a 45° angle along theirsides s₂ at a distance l₂ =27.3 mm from the front surface 54 and thefront surface (not shown) formed by step (i) above, respectively; and(iii) slicing each of the four resulting substrates in half (at a 90°angle) along the front surfaces at a distance h₁ =50.14 mm from thebottom surfaces.

Referring to FIG. 4, a lens array mold 60 is used to construct thelenses arranged along the front surfaces 44 of the substrates 40. Thetop surface 62 of the mold 60 has a plurality of lens cavities 64 formedtherein. The mold is preferably constructed from a glass substratematerial having resin adherence properties different from that of theglass material used for the substrates 40. The mold is adapted forinsertion into a fixture having adjustable references when mated to thesubstrates, as described below.

Specifically, five pairs of overlapping cavities 64 are formed in thesurface 62 using a diamond-tipped, precision cutting tool (not shown).For a particular lens, each cavity 64 has a diameter d of 3.1 mm and aradius of curvature of 13,381 mm. The spacing d₁ between the centers ofthe cavities of an overlapping pair is 2.94 mm and the distance d₂between the centers of cavities of adjacent pairs is 7.86 mm. Thesedistances are determined by the spacing between the recording disksurfaces of adjacent disks and the recording disk surfaces of each disk,respectively. Although the diamond-tipped cutting tool creates agenerally smooth, inner surface of the cavities 60, the inner walls 66are thereafter mechanically lapped to ensure a very smooth finish. Thisis because imperfections in the cavities will be impressed on the lensesduring the fabrication process. Therefore, the initial process offorming the mold may be time consuming because of the requiredprecision, but the resulting mold 60 may be used many times duringsubsequent "lens replication" processes.

The mold 60 is then placed into a mating fixture 70, shown in FIG. 5,that is configured and arranged to provide pressure contact between thesubstrate 40 and mold 60. It is to be understood that the elements inFIG. 5 (and FIGS. 1-4 and 6-8) are somewhat exaggerated and are notdrawn to scale for purposes of ease of depiction and ease ofdescription. Specifically, no attempt has been made to depict the exactspacing between elements or the exact dimensions of the elements,although the drawings depict the relationship relative to one another.The fixture 70 includes a plurality of fixed arms 72 arranged to contactthe mold 60 and substrate 40 at a predetermined reference plane.Adjustable arms 74 are provided to secure these pieces in mating contactrelation to one another.

The mold cavities 64 are then filled with an optical-quality resin 76,such as clear epoxy material, which may be curable by ultra-violet (UV),as well as thermal, radiation. The substrate 40 is placed into thefixture 70 and pushed against the fixed arms 72 to establish, as noted,a defined reference plane for the front surface 44 relative to thecavities 64 of the mold 60. The adjustable arms 74 are then extended tocontact the substrate 50 at the defined datum points 25 (FIG. 2) toensure alignment of the substrate against the mold cavities 64. As shownin the drawing, a screw-type pushing tool may be used as an adjustablearm 74 to apply pressure at a datum point.

The datum points 25 are arranged such that there is a small gap gbetween the front surface 44 of the substrate and the mold 60; asdepicted in FIG. 6, the gap g allows the epoxy 76 to slightly overflowbeyond the diameter of the cavities 64 to enable adjustments andcompensation during alignment of the mold 60 to the substrate 40.Referring again to FIG. 5, once the positions of the adjustable arms 74are established, they are locked to secure the substrate 40 against themold 60. The epoxy is then cured.

The mold is thereafter separated from the substrate leaving a pluralityof precisely-molded, replicated lenses 30 having a predetermined shapeand height on the front surface 44 of the substrate 40, an example ofwhich is shown in FIG. 7. Specifically, the geometry of the lenses 30compensates for any differences in the indices of refraction of thelenses and substrate materials. In the illustrated example, thethickness t of each replicated lens 30 is approximately 0.25 mm. Asnoted, the substrate and mold materials are chosen such that the epoxyadheres to the substrate and not to the mold.

Refer now to FIG. 8. The front surface 44 (hereinafter 22 as designatedin FIG. 2) and the longer side 42 (now designated 36) are coated withanti-reflective material, e.g., magnesium fluoride (MGF2). The 45°surface 26 (FIG. 2) is coated with a multi-layer dielectric material,such as titanium dioxide (TiO₂) and silicon dioxide (SiO₂), thatprovides greater than 99% reflection of an incident beam whileminimizing the phase shift of the beam. However, the 45° surface 26 maynot need the reflective coating because of the excellent optical qualityof mirror due to the principle of total internal reflection, i.e., theangle of incidence at the 45° surface is greater than the critical angleof the material. This completes fabrication of the LMT 20.

The invention provides a high-precision lens/mirror tower capable ofhigh-volume, repeatable production and at low-cost. The monolithic LMTarrangement integrates the functions of individual mirrors with asingle-surface lens array, thereby ensuring alignment between the lensand mirror elements, while increasing the accuracy of the tower byreducing the number of discrete components. The common lens surface alsoallows for overlapping lenses needed to precisely direct a laser beam toobjective lenses located between closely-situated disks.

The foregoing description has been directed to specific embodiments ofthis invention. It will be apparent, however, that variations andmodifications may be made to the described embodiments, with theattainment of some or all of their advantages. Therefore, it is theobject of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

What is claimed is:
 1. A component of a storage device having aplurality of optical disks, said component comprising:an optically-clearsubstrate including a front surface and a back surface oriented at anangle relative to said front surface, said angular back surface servingas a mirror to direct an optical beam to any of said optical disks; anda plurality of lenses affixed to said front surface of said substrate,said lenses arranged for passing said optical beam to said angular backsurface of said substrate and onto said disks.
 2. The component of claim1 wherein said angular back surface is oriented at a 45° angle relativeto said front surface.
 3. The component of claim 2 wherein saidoptically-clear substrate is a rectangular parallelopiped except forsaid angular back surface.
 4. The component of claim 3 wherein saidsubstrate is composed of glass material.
 5. The component of claim 1further comprising means, applied to said angular back surface, forminimizing phase shift of said directed optical beam.
 6. The componentof claim 5 further comprising means for positioning said componentwithin the storage device.
 7. The component of claim 6 wherein saidlenses are arranged as overlapping pairs of said lenses.
 8. Thecomponent of claim 7 wherein said lenses are composed of anoptical-quality resin.
 9. Apparatus of a storage device having aplurality of optical disks with recording surfaces for storing data,said apparatus comprising:an optically-clear base substrate having atrapezoidal configuration with a generally planar front surface and anangular back surface having a plurality of mirror elements integrallyformed thereon, said mirror elements configured to direct an opticalbeam to any of said optical disks; means, applied to said angular backsurface, for minimizing phase shift of said directed optical beam;means, affixed to said substrate, for positioning said apparatus withinthe storage device; and an array of lenses affixed to said planar frontsurface of said substrate, said lenses having positions relative to oneanother to enable passing of said optical beam to said angular backsurface of said substrate and onto said recording surfaces of any ofsaid optical disks.
 10. The apparatus of claim 9 wherein said minimizingmeans comprises a multi-layer dielectric material.
 11. The apparatus ofclaim 10 wherein said positioning means comprises a plurality of datumpoints.
 12. The apparatus of claim 11 wherein said plurality of mirrorelements are configured as one reflective surface of said substrate.